Process for developing electrostatic charge patterns



Oct. 14, 1969 P. M. CASSIERS ET AL 3,472,676

PROCESS FOR DEVELOPING ELECTROSTATIC CHARGE PATTERNS Original Filed Nov. 18, 1965 2 Sheets-Sheet l F/G.3 I FIG. 4

INVENTORS PAUL MARIA CASS/ERS ANDRE ROTT LOUIS ACHILLES MEEUSSEN JOZEF LEONARD VAN ENGELAND BY WATSON, COLE, GRINDLE I WATSON ATTORNEYS Oct. 14, 1969 P. M. cAssIERs ETAL 3,47 76 PROCESS FOR DEVELOPING ELECTROSTATIC CHARGE PATTERNS Original File ad Nov. 18, 1965 2 Sheets-Sheet FIG. 7a FlG.7b F/G.7c

INVENTORS PAUL MAR/A CASS/5R5 ANDRE ROTT LOUIS ACH/LLES MEEUSSEN JOZEF LEONARD VAN ENGELAND BY WA TSON,COLE,-GRINDLE & WATSON ATTORNEYS United States Patent ABSTRACT OF THE DISCLOSURE Electrostatic charge images are developed on a hydro- I phobic surface with a conductive, polarizable, aqueous liquid developer contained in an applicator consisting of closely spaced capillary passages. The ends of the capillary passages contact the hydrophobic surface and only permit the liquid to flow therefrom in response to the electrostatic charge on the surface. An external electric field can be applied to aid in the development. The capillary passages are connected to a liquid supply to provide automatic replenishment.

This invention relates to the development of electrostatic charge patterns.

This application is a continuation of U.S. patent application Ser. No. 508,485, filed Nov. 18, 1965, now abandoned, which was in turn a continuation-in-part of U.S. patent application Ser. No. 150,820, filed Nov. 7, 1961, and now abandoned.

Many images and signal recording processes are known which involve the creation of an electrostatic record from or by means of which a visible record can subsequently be formed. We refer by way of example to electrostatic printing processes (see, e.g., United Kingdom patent specification No. 618,512), electrostatic recording processes, xerography (see, e.g., United Kingdom patent specification No. 672,767), the Electrofax process as described by C. J. Young and H. C. Greig in RCA Rev. (1954), 469, electrothermography as described by P. M. Cassiers, J. Soc. Phot. Sci. Eng., 4 (1960), 199.

Many methods of developing latent electrostatic records have been proposed. We mention in particular, methods in which the electrostatic record is developed to a powder image by the electrostatic attraction of powder dusted over the electrostatic record or by the deposition of powder by electrophoresis from dispersion in a liquid with a high electric resistivity.

Although powder development gives good results, it is open to various objections. These arise in part from the inconvenience of working up siutable dry powders and the necessity for a special fixing step after deposit of the powder, e.g., heating or application of solvents, so as to make the image permanent. Moreover, in order to obtain uniform and equivalent image reproduction with powder development, a rather extensive and expensive installation has to be used.

When developing with dispersions in liquids with high electrical resistivity, the choice of suitable liquid media is limited. Electrically insulating liquids have to be used.

The present invention is based on the discovery of a new method of utilising electrostatic records to produce records in diiferent kind, e.g., visible records.

According to the present invention, contact or close proximity is established between a hydrophobic surface of a charge carrier bearing an electrostatic charge pat 3,472,676 Iatented Oct. 14, 1969 tern detectable at such surface, and a liquid supply means comprising at least one capillary passage in which a quantity of an aqueous liquid medium (which may be water alone) which wets the wall(s) of said passage is held in close proximity with said surface and by which such surface is normally substantially unwettable is held, and under the influence of the electrostatic charges borne by said carrier liquid medium is attracted from said capillary passage and deposits on and wets said surface in accordance with the said. charge pattern.

It has been discovered that electrostatic charges on a charge carrier having a hydrophobic surface are capable of influencing the interfacial tension between the hydrophobic surface and a strongly polarizableliquid medium having a high dielectric constant so that due to such charges the surface can be wetted by a liquid by which the surface is normally substantially unwettable.

The expression hydrophobic where used in this specification in relation to a charge carrier surfaec means that water forms on the said surface (in the absence of attractive electrostatic charges) a contact angle of at least 90. (For a definition of contact angle see J. Alexander, Colloid Chemistry, vol. I, Principles and Applications, 4th ed., D. van Nostrand Company, Inc., New York, pp. 79-80.)

In carrying out the process according to the present invention, a liquid medium is used which does not normally wet the hydrophobic surface of the charge carrier, or not to any appreciable extent, but which will wet such surface under the influence of the encountered electrostatic attractive charges. In the interest of forming good quality records it is preferable for the electrostatic charges to be capable of reducing the contact angle between the liquid and the hydrophobic surface to such an extent that complete or spreading wetting occurs (contact angle of 0 or approximately so). Water alone can be used, or water together with one or more other ingredients (e.g., an organic polar liquid), provided that the composition as a whole has the requisite properties referred to. When using a mixture of liquids, the mini mum proportion of water required will depend upon the properties of the other ingredients. If an organic liquid with high dielectric constant such as formamide is incorporated, this may be present in substantial amount, e.g. 60% by weight, but in general it is preferred in all cases to use a liquid comprising at least 60% by weight of water. As will hereafter be illustrated, the liquid used in the process can be a solution. It is also possible to use a liquid which contains dispersed material, e.g., to use a liquid in which solid material is dispersed, forming a stable suspension.

The expression electrostatic charge pattern is used broadly to include any distribution of charges within a support area such that different parts of this area are charged to different extents or such that some parts only are charged. Thus the expression includes electrostatic images of reading matter, diagrams, pictures etc., and charges constituting the electrostatic record of wireless or other signals.

The liquid supply means used in the performance of the present invention may have any shape of form provided that it provides at least one passage which is capable of holding a quantity of liquid adjacent the surface onto which the liquid is to be attracted when the supply means and said surface are brought together into contact or virtually so, the said passage being of such dimension that it exhibits a capillary effect (capillary raise or suction of liquid).

In a simple embodiment of the invention the liquid supply means comprises a single capillary passage, formed by the narrow space between two plates or by a slit in the bottom of a container, but a variety of other supply means providing capillary passages open at both ends or capillary recesses e.g., grooves may be used. Examples of different types of capillary media enabling automatic feed of the liquid therein from a reservoir are: fabrics, porous ceramics and sintered materials, e.g., in the forms of flat plates or cylinders. Examples of capillary media containing capillary passages of different types are:

Fabrics: silk gauze, polyamide fibres, glass fibre, stencils impregnated papers, felt, leather, bundles of fibres or capillaries stuck together, fur and brushed.

Ceramic materials: porous natural or artificial stone, unglazed porcelain.

Sintered plates and perforated plates: glass sinters, porous sintered polyethylene and polytetrafluoroethylene, sintered steel, sintered stainless steel.

The geometric form of the capillaries is not critical. For example the capillaries can be cylindrical, hexagonal or irregular. Favourable results have been obtained with capillary passages of a width from 30,11. up to 1 mm.

In general, and in particular when actual contact is to be established between the electrostatic charge carrier and the capillary applicator, the extreme surface or surfaces of the applicator (i.e., those surfaces which will be against or nearest to the charge carrier) should preferably be formed of material which is not wetted by the used liquid and which is electrically insulating e.g. resin or glass, and the number of tangent or contact points of this surface or surfaces is preferably restricted to a minimum. These measures contribute to keeping the image background clean and preventing premature dissipation of the electrostatic charges.

In a particularly favourable embodiment the liquid applicator device comprises a roller with capillary passages in its surface. In case of a bimetal surface the passages e.g., consist of a hydrophilic material such as chromium and the fringe of their openings consist of a hydrophobic material such as copper or a hydrophobic resin.

If desired the said roller may be provided with certain letters, signs, lines or figures which serve to print the charge carrier with standard reading matter or data regardless of the electrostatic charge pattern. For example, the applicator may serve to print letter heading, form columns or other standard form features, or calibrations for curves such as those used in oscillography and other curve-recording systems.

The new technique of depositing liquid in dependence on an electrostatic charge pattern may be applied to give a one-step development process in which can be created, by the application of developing liquid, a visible image which does not require a special after treatment for fixing purposes. Thus, for example, the aqueous liquid medium used in a process according to the invention may be an ink or dye or a liquid medium which contains an ingredient which reacts, e.g., with a component present in the charge carrier or with the atmosphere, to form a dye. The invention is not however limited to the direct production of dye or other visible or final records from an electrostatic record. The new technique opens the way to a'variety of alternative ways of using such a record. As one example, after depositing an uncoloured hydrophilic composition on the hydrophobic charge carrier surface under the influence and according to the pattern of electrostatic charges thereon the resulting liquid deposit can be converted into a readily visible form by passing over the surface a roller charged with an aqueous dyestutf solution. It is also to be understood that aqueous liquid medium deposited under the influence of the electrostatic charge pattern can be transferred to another support for the purpose of producing a visible record on that support in the same or by a subsequent step.

The particular method by which the electrostatic charge pattern is formed is not in any way crucial. By way of example the techniques for producing electrostatic records as summarized in the introductory paragraphs of this specification can be used, or any other method, e.g., electrical polarization of an insulating material. The invention has so far however been primarily developed in connection with electrophotographic processes in which the electrostatic record is created in a photoconductive layer as a result of the application of an electric charge or field and record-wise exposure of the layer to light or other radiation which raises the electrical conductivity of the layer.

The deposition of liquid on a surface according to the electrostatic charge pattern may takeplace Within a direct current electric field extending substantially uniformly across the surface of the charge carrier and having a direction, normal to such surface, which is opposite to that of the field due to the electrostatic charges constituting the latent record. Such a superimposed field may be created by causing a source of E.M.F. to apply a bias potential to the liquid supply means. By using such a superimposed electric field better quality visible images cari be obtained, in particular images with better contrast.

The application of the invention as above defined and described results in the liquid being only deposited or deposited to the greater extent on the charged or more heavily charged areas of the charge carrier. By a small modification of the procedure, aqueous liquid medium can be caused to deposit so that the wetted or more strongly wetted areas of the charge carrier are the uncharged areas or the areas bearing a relatively small charge. For example, this result can be brought about by superimposing a sufficiently high voltage direct current electric field which extends substantially uniformly across the surface and has the same direction, normal to the charge carrier surface, as that resulting from the electrostatic, charge pattern. The invention includes any such modified process. When such a modified process is performed, selective or differential wetting of the charge carrier; according to the electrostatic charge pattern takes place, in the sense that the deposited liquid pattern visibly or detectably records the subject of the electrostatic record, but the liquid and electrostatic records do not correspond in the sense that two positive records of a text correspond; rather one is positive and the other negative. The expression counter image is used herein to denote an image which is reversed in this sense with respect to the electrostatic image. Counter image can also be obtained by an alternative modification, namely by the superimposition of an alternating current field as will hereafter be described.

It preferable for the electrostatic charges to attract the liquid against the action of some restraining force. By way of example the charge carrier and the holder for the aqueous liquid medium are related so that the liquid medium has to rise onto the charge carrier; in this case the gravitational force acts against the attraction of the liquid medium onto the carrier. As an alternative or in addition the electrostatic attraction may take place in a superimposed electric field as already referred to, acting contrary to the direction of the field due to the electrostatic. charge pattern. The imposition of such restraint leads to better visible image quality and in particular better image definition.

As a carrier for an electrostatic charge record to be used in a process according to the invention, insulating layers or sheets containing photoconductive or thermocofidlictive substances may be used. These materials preferably comprise a back-layer or a support which possesses higher conductivity than the insulating layer.

According to a preferred feature, a photoconductive layer with hydrophobic properties is used as carrier for the electrostatic charge pattern. Preferably the photoconductive layer comprises a photoconductor dispersed in an insulating binding agent. The usual photoconductive layers which comprise organic or mineral photoconductive substances incorporated into an insulating polymeric binding agent, and the photoconductive layers which comprise organic photoconductive polymers, possess a hydrophobic character. Examples of photoconductive layers comprising photoconductive polymers are described, e.g., in United Kingdom patent specifications Nos. 964,877 and 964,875. Examples of photoconductive binding agents more especially for photoconductive zinc oxide are described in United Kingdom Patent No. 964,885.

As an illustration, a non-limiting list of hydrophobic polymeric binding agents for photoconductive materials is given here:

polyvinylacetate copolymer of vinyl acetate and an ester of vinyl alcohol and a higher aliphatic carboxylic acid such as lauric acid, stearic acid, palmitic acid, e.g., co(vinyl acetate/ vinyl stearate) (85/ polyalkylmethacrylate, e.g., Plexigum P26 (trademark for an acrylic resin marketed by Rohm & Haas G.m.b.H., Darmstadt, Germany) Kunstharz EM (trademark for a ketone resin prepared by the condensation of an aliphatic ketone with formaldehyde, marketed by Rheinpreussen G.m.b.H., Homberg, Germany) esterified copophony Syntex 800 (trademark for a 100% cyclic rubber from the N.V. Chemische Industrie Synres, Hoek van Holland, Netherlands) Polyol X-450 (trademark for a copolymer of the following formula:

I L oHa Hi I The ratio of insulating binding agent and photoconductor depends on the required quality of the photoconductive layer with respect to the photoconductive properties, mechanical strength and insulating power. Good results are obtained with a ratio of binding agent and photoconductor of 1:3 to 1:9. When using layers with a relatively high content of binding agent, the image sharpness diminishes and when using layers with a much lower content of binding agent, the relaxation time of the charge diminishes quickly.

To layers containing binding agents which are not sufficiently hydrophobic by themselves, water-repellent properties can be conferred as known per se by special addltives or by an after-treatment. Additives enhancing the hydrophobicity, e.g., stearic acid as described in the United Kingdom patent specification No. 883,783, or binding agents bearing active hydroxyl groups which react with di-isocyanates, as described in the United Kingdom patent specification No. 896,610, can be incorporated in the photoconductive layer.

If polystyrene-butadiene latexes are used as binding agents, a thermal after-treatment can be applied as described in the United Kingdom patent specification No. 766,979.

The surface of the photoconductive layer can also be made more hydrophobic by adsorption of an appropriate substance on the photoconductive layer.

It is also possible to make the surface of a photoconductive layer more hydrophobic by applying a coating layer comprising a hydrophobic layer-forming material such as a hydrophobic polymer, varnish or wax.

If the photoconductive layer is more hydrophobic than is required, a layer (which may be very thin) of a less hydrophobic substance can be applied thereon or the surface can be modified by applying a sufiiciently small quantity of a hydrophilic colloid, e.g., gelatin, polyvinylalcohol, a cellulose derivative or an alginic acid derivative. The thickness of such hydrophilic layer preferably varies between 0.2 and 2,4/.. This layer can also be applied from a strongly diluted solution of a wetting agent, but should not enable the surface charge to be carried off. The photoconductive layer and/or a top layer for increasing the hydrophobic properties may include other known additives such as plasticizers, dispersing agents, optical bleaching agents, substances counteracting oxidation and ageing, agents improving the gloss, matting agents, sensitizing dyes and chlorine containing polymers which increase the sensitivity as described in our United Kingdom patent specification No. 964,878. The amount and the nature of these substances are chosen in such a way that the chargeability of the layer is not markedly diminished.

In the course of our experiments it has been found that particularly good results can be obtained in the formation of a liquid record by means of an electrostatic charge record if the electrostatic charge carrier is a photoconductive layer comprising photoconductive zinc oxide as a photoconductive component. The photoconductive zinc oxide/binder composition preferably consists for at least 50% of zinc oxide in a hydrophobic binder. Specially good results are obtainable when the zinc oxide has been treated with a suitable acid compound or salt which increases the dark-resistivity of the zinc oxide. The increase of the dark-resistivity of the photoconductive zinc oxide permits the formation of a photoconductive layer which comprises the zinc oxide dispersed in a binder and in which the specific resistivity of the binder is not markedly higher or is even the same as or lower than that of the acid-treated zinc oxide.

By usual photoconductive zinc oxide is understood any commercially available type of zinc oxide which has been prepared according to the French process; in the process zinc oxide is prepared by oxidation of zinc vapour.

Zinc oxide types which appear from our experiments to give particularly good results are, e.g.: Blanc de Zinc, Neige Extra Pur, types A, B and C, marketed by Vielle Montague S.A., Liege, Belgium, Zinkoxyd (reinst) marketed by E. Merck A.G., Darmstadt, Germany, Florence Green Seal Lead-Free Zinc Oxide marketed by the New Jersey Zinc Company, New York, N.Y., U.S.A., Zinc Oxide Analytical Reagent, marketed by Mallinckredt Chemical Works, St. Louis, Mo., USA.

The preferred compounds with acid properties for treating the photoconductive zinc oxide are:

Aliphatic non-substituted monoand dibasic carboxylic acids or aliphatic monoand dibasic acids which may contain one or more hydroxyl groups such as lactic acid and tartaric acid, and organic phosphorous compounds corresponding to the general formulae:

wherein:

R represents a hydrogen atom, a hydroxyl group or a chlorine atom,

R represents a hydroxyl group, a chlorine atom, an alkyl group, a substituted alkyl group, an aryl group, a substituted aryl group, an alkoxy group, a substituted alkoxy group, an aryloxy group or a substituted aryloxy group, and

R represents an alkyl group, a substituted alkyl group, an aryl group, a substituted aryl group, an alkoxy group, a substituted alkoxy group, an aryloxy group or a substituted aryloxy group.

In the foregoing description we have referred to the formation of liquid records under the influence of an electric field. Instances have been given wherein a bias potential is applied to create a direct current field in the same direction as the electrostatic charge field or in the opposite direction. It is also possible, provided the electrostatic charge carrier has rectifying properties to deposit the liquid in an alternating current field maintained normally to the charge carrier surface and this may create an effective bias which assists or retards wetting of the charge carrier depending on the size of the electrostatic charges and the composition of the carrier. When using a carrier comprising photoconductive zinc oxide a reversed or counter image is obtained while with certain other materials a non-reversed image of improved quality is surprisingly obtained. It is preferable to use an alternating current field with .a frequency substantially above the normal 50 cycles, say a frequency of 100 cycles or more, and we have so far obtained the best results with frequencies above 500 cycles, e.g., 1000 cycles.

As will be clear from examples of the invention to be given hereafter it is not necessary for the whole charge carrier surface to be simultaneously contacted or almost contacted by the liquid supply means. The latter can, e.g. be in the form of a roller which moves progressively over said surface. In such cases, when the deposition of liquid takes place in a superimposed electric field, the latter need not extend over the whole of the surface area containing the electrostatic charge pattern during the whole of the period in which liquid is being deposited but can at any one moment merely extend substantially uniformly across the area of the charge carrier surface at which liquid is presently available for deposit and the said field may move progressively along the said surface with the supply means. The claims hereof are to be construed accordingly.

The following further elucidation of the invention includes a description of further factors which can usefully be taken into account in preparing the electrostatic charge carrier and in selecting the composition of this carrier and of the liquid.

As described on pages 394-395 of the aforesaid book of Schwartz, Perry and Berch, it is explained that there is a relation between the roughness of the surface of a solid material and the contact angle with a particular liquid. It appears therefrom that size of the contact angle, in other words, the wetting power, may be influenced by adapting the roughness of the surface.

The degree of roughness in the structure of the surface of a charge carrier in the form of a photoconductive layer comprising a dispersed photoconductor may be influenced by suitably choosing the grain size and the degree of dispersion of the photoconductive material or by the controlled floculation of the binding agent in some solvents. Further, a granular structure of the surface can be obtained by including compounds for this purpose in the composition of the photoconductive layer, e.g., by appropriately dispersing a non-photoconductive compound in a continuous phase formed by a photoconductive polymer as described in the Belgian patent specification 609,056.

Further a certain rough surface structure can arise by applying the photoconductive layer onto a screened support. Surface roughness can also be obtained by applying the photoconductive layer in a regular or irregular screen form, as, e.g., by applying with a screened roller or by pressing a screen profile into the half-dry or plasticized thermoplastic material of the photoconductive layer or of any applied layer giving a more hydrophobic surface.

The surface roughness of a layer, i.e., the degree of the unevenness, may be determined with a Perth-O-Meter (ref. Dr. Perthen, Hannover/Hemmel-Werke, Mannheim). It is expressed by the sum of two terms, namely W+Rt wherein W (Welligkeit) is the depth measure for the macro-unevenness of the surface and Rt is the depth measure of the micro-unevenness of the surface. Macrounevenness here refers to the deviations of the general surface contour from a plane and micro-unevenness refers to the surface irregularities which are present locally in successive parts of the surface contour.

The total value W+Rt of the surface roughness is preferably comprised between 2 and 15 1.-

The degree and kind of surface unevenness which is permissible depends in part upon the way in which liquid is to be made available for wetting the surface. The liquid must have proper access to all parts of the surface. When the liquid is applied from a roller onto the charge carrier attention should be paid to the distance between the free liquid surface on the Wetting device and the deepest point in any recesses or depressions or reticulations in the surface to be developed. Good results have been obtained with distances comprised between 1 and p, preferably between 3 and 30 If the distance is taken too small, e.g., by exerting too strong a pressure, the charge image is destroyed. If too big a distance is taken, the fine image details are lost. The adjustment of the suitable distance between the liquid surface and the profile of the material depends on the surface structure of the material and the kind of the developing liquid. The optimum value of surface roughness for obtaining a favourable contact angle is shifted to higher or lower values, according to whether the wetting with liquid is facilitated or inhibited by one or more of the following factors: viscosity and surface tension of the liquid, surface roughness and surface tension of the material to be developed, interfacial tension of the insulating material/liquid system, the quantity of the conveyed liquid, the pressure exerted upon the liquid, gravity, capillary forces, streaming potential and magnetic and electrical forces which arise between the material to be wetted and the wetting liquid.

As aforesaid, the contact angle formed by the developing liquid with the uncharged insulating material, has to be such that liquid on or opposite to the uncharged areas or on or opposite to the image areas with insufiicient field strength does not or does not sufiiciently undergo electrostatic influence which causes spreading out or wetting, of the surface. On the other areas, on the other hand, the electrostatic field at the surface of the insulating material performs the work necessary for wetting.

The work required for wetting can be increased or decreased by modifying the physical properties of the liquid and of the insulating material to be wetted, e.g., by modifying: surface tension, electric conductivity, polarizability, capacity or viscosity.

In order to obtain a visible image, the aqueous liquid medium may contain preferably in solution or in dispersion dyestuffs which are fast to light. Agents for improving the fastness to light of the dyestuffs and mordanting agents may be added. Further, compounds may be added to the liquid which improve dissolving, emulsifying and dispersing of substances which influence the physical and chemical behaviour of the liquid. These substances comprise organic or mineral dyestuffs, substances which increase or decrease the surface tension and/or the conductivity and/or the polarizability and/or the capacity and/or the viscosity, binding agents, e.g., colloids and latexes, macromolecular compounds, substances improving the fastness to light of the dyestuffs, substances preventing the ink image from drying quickly.

For the latter purpose, a hydroscopic substance is preferably used if the liquid medium is water.

In another embodiment an aqueous developing liquid is used which contains no dyestuff but a colourless or little coloured component, which is converted into a dyestuif only during or after the development, by oxidation with atmospheric oxygen, by exposing to light or heat, or by reaction with a reaction partner which is present in the support of the electrostatic image or (in case of transfer) in the ultimate support.

As we have already explained, processes according to the invention are preferably performed using an aqueous medium consisting for at least 60% by weight of water. Very suitable developing liquidscomprise water and one or more compounds of the following categories in the proportions stated. 1

(1) 03-20% of a dycstutf soluble or dispersible in water.-Suitable organic dyestuif are e.g.:

- (3.1. Crystal Violet 42,555 Malachite Green 42,000 Methylene Blue 52,015 Victoria Blue 42,595 and 44,045 Carmine Red 75,470 Nigrosine C 140 powder 50,420 Chloramine Black EX (Dark) 30,235 Rayon Black C (double conc.) 35,255 Chris Cuprofies 3 LB Direct Black 63 Suitable inorganic pigments are any structural form of carbon, e.g., graphite, carbon black, lamp black, bone black, charcoal, ultramine blue, cadmium sulfide, titanium dioxide, zinc oxide, iron oxide, magnetic iron oxide, aluminum powder and bronze powder.

(2) Colourless or little coloured compounds convertible to a coloured compound.St1ch a colourless or little coloured compound can be converted into a coloured compound, during or after wetting the charge carrier, with a substance present in or on the material to be developed. Suitable reaction partners the reactivity of which is promoted by the presence of a liquid phase are, e.g., those described in 'United Kingdom patent specification No. 898,354.

Suitable known colour reactions in aqueous medium are e.g., coupling reactions of diazonium compounds with known coupler compounds such as p-naphthol.

For colour formation in situ ferric salts are suitable which react with aromatic compounds containing hydroxyl groups, e.g., pyrogallol and dodecyl gallate.

Suitable colourless reaction partners are the colourless triazolium and tetrazolium compounds such as those described in United Kingdom patent specification No. 670,883. These compounds are converted into a coloured compound by a reducing reaction partner.

The classical colour coupling reactions between oxidizable aromatic amino-developers and colour couplers are also applicable. Classical colour coupling reactions are described, e.g., in The Theory of the Photographic Process by C. E. Kenneth Mees, revised ed., 1954, The Macmillan Company, New York, pp. 584-589.

Many other colour reactions and bleaching-out reactions of colours, which also may be used for forming an outlined image pattern, are known to those skilled in the reproduction art. Further, an extensive list of colour reactions is set out by Feigl in SpotTests, 1954, Elsevier, Publish. Corp., Amsterdam.

It is also possible, provided a suitable solvent or dispersing agent for polyvinyl chloride is used, to cause a colour reaction with zinc oxide, after the deposition of a polyvinyl chloride residue on the charged areas of a layer containing zinc oxide by warming the polyvinyl chloride.

It is evident that an applied colour reaction may be activated or accelerated by heat and light, and that in the layer with the electrostatic pattern as well as in the developing liquid catalysts known per se can'be incorporated which promote the colour reaction, if necessary.

The formation of a visible image by reacting of one or more reaction partners in the material surface to be developed, and which is attainable for the reaction with one or more reaction partners in a liquid phase, has the advantage that the formed image is well anchored in the surface of the material so as to be very resistant to mechanical erasure.

(3) 02-20% of a substance influencing the surface tension.-The surface tension may be increased by adding water-soluble substances, e.g., potassium carbonate, aluminum sufate, iron sulfate, cadmium chloride and magnesium sulfate. A list of other such substances is given, e.g., in Taschenbuch fur Chemiker and Physiker by J. DAns and E. Lax, edition 1949, Springer Verlag, p. 1008.

The surface tension may be decreased by adding watermiscible organic substances, e.g., methanol, ethanol, acetane, methyl ethyl ketone, acetic acid, hydroquinone, lauryl sulfonates, dodecyl sulfonates, saponine and polyglycol derivatives.

Other useful surface-active substances are given in Textilhilfsmittel und Waschrofie by K. Lindner, Wiss. Verlagsgesellsch, m.b.H., Stuttgart, 1954.

(4) Up to 10% of a substance which delays drying up of the ink.E.g., glycerol, glycol and sorbitol.

(5) Up to 10% of a water-soluble or water-dispersible binding agents.-E.g., gum arabic, carboxymethyl cellulose, casein, polyvinylpyrollidine, polyacrylates, polystyrene, polyvinylacetate, waxes, slicates and colloidal silicic acid. These substances decrease liability of the dry ink to be washed away by water.

(6) Up to 35% of an organic polar liquid with high dielectric constant and miscible with water.E.g., formamide.

According to the choice of the used dyestuffs (if any) in the insulating layer and in the ink, different colour contrasts may be obtained. If the colour of the ink is darker than that of the insulating layer, a positive image is obtained. If the colour of the ink is lighter than that of the insulating layer a counter image (positive print of a negative original) is obtained. In the latter case, a photoconductor which is dark-coloured in itself is used e.g., selenium, cadmium sulfide, cadmium selenide and antimony sulfide, or a white photoconductor e.g., zinc oxide, to which sensitizing or other dyestuffs are added. Developing is carried out with a dispersion of a white or clear-coloured pigment, e.g., titanium oxide, zinc oxide, zinc sulfide, barium sulfate, antimony oxide, china clay and calcium carbonate.

With reference to embodiments of the invention in which the electrostatic charge carrier is a photoconductive material, this may incorporate an integral or attached conductive support for the photo'conductive layer. Suitable conductive supports are, e.g., plates or foils of metal such as aluminium, copper, bronze, lead and zinc, or glass-plates provided with a thin layer of tin oxide having a specific resistivity of 10 to 10 ohm cm., foils or fabrics of plastic substances, provided with a thin conductive layer such as described in United Kingdom Patent No. 995,491. Other suitable kinds of paper Suitable kinds of paper are those which show a resistivity lower than 10 ohm cm. at a relative humidity of 50% e.g., the kinds of paper described in United Kingdow Patent No. 995,491. Other suitable kinds of paper are those containing at least 2% of conductive filling materials, e.g., carbon. Other suitable papers are those of which the surface, which is turned to the photoconductive layer, is provided with a conductive layer, e.g., a thin lead or aluminium foil, or a dispersion of a metal powder or of carbon powder in a binding agent. Finally the fabric consisting preponderantly of carbon, e.g., those marketed by Union Carbide Corporation, New York, N.Y., U.S.A. under the name Graphite Cloth 1 1 Grade WCA and WCC, may successfully be used as conductive supports.

Fabrics or kinds of paper which show no sufficient conductivity at the prevailing air humidity, may also be used successfully, provided that the back-side of the support is wetted with water before or during the development.

Images formed in experiments using a capillary system according to the invention have excelled by their sharp contrast, good coverage, of large black areas, and by a clear image background. The images were free of the faults which are associated with many existing developing methods, e.g., fringing effect and halation round the image.

With regard to the mechanism of processes according to the invention, it is assumed that an electro-osmotic phenomenon is in question, without limiting, however in any way, the developing liquid, the material to be developed, the developing devices nor the application of this process for selective electrostatic wetting.

By electro-osmosis (see Kolloidchemischer Praktikum, Dr. E. Sauer, Berlin Verlag von Julius Springer (1935), pp. 697 1, especially FIG. 27 on p. 70) is meant a phenomenon whereby a liquid is moved under the influence of an electric potential. This differentiates it from the socalled electrophoresis which has already been described in the existing liquid developing systems, and according to which dispersed particles move in an electric field. In the known electrophoretic developing method, the liquid is brought into direct contact with the electrostatic image, and in order to prevent the electric charges from flowing away, this liquid must have a high electric resistivity. In carrying out the present invention, this is by no means necessary, and favourable results are obtained even with aqueous solutions of salts which have a high specific conductivity, e.g., higher than 10- mho/ cm.

The invention will be further described with reference to certain embodiments illustrated by the accompanying diagrammatic drawings in which:

FIG. 1 is a representation of an electrostatic charge pattern.

FIG. 2 represents a developing device using a capillary membrane.

FIG. 3 represents the developed image.

FIGS. 2a and 2b are enlarged sectional views of the membrane of FIG. 2.

FIG. 4 represents the transfer of the developed image onto another support.

FIGS. 5 and 6 represent an apparatus for progressively providing developing liquid from a roller containing capillary passages.

FIG. 7 represents stages in the formation of a reversed record (counter image).

In FIG. 1 a schematic representation of an electrostatic charge pattern is given, which is obtained in a known electrographic, electrophotographic or electrothermographic way.

FIG. 2 represents a process in which a container 11 is filled with an aqueous ink 4, the container being closed at its top by a capillary membrane 13 with capillary openings or passages therein and at its bottom side by a movable closure 14. During development the closure 14 can be moved to keep the liquid level up to the mark in the capillary openings 15 in the membrane 13. Moreover, by adjusting the pressure on closure 14, the angle of the liquid meniscus in these capillaries and hence the force needed for drawing out the liquid, may be regulated. FIGS 2a and 2b show the meniscal form of the ink in the capillaries 15 of the membrane 13 at relatively high and low pressures respectively. By this means the gradation and developing speed can be influenced.

FIG. 4 shows the transfer of the ink-image of FIG. 3, while still wet, from the insulating support 5 onto a 12 porous support 16 e.g., paper by means of a pressure roller 17.

.By means of the precesses described and illustrated, positive prints are obtained from a positive original. It is also possible to make positive prints from a negative original, e.g. by using as charge carrier and insulating layer which comprises at least at its surface a dark dyestuff, and by using as developing liquid a solution of a bleaching agent for this dyestuff.

Printing plates can also be manufactured according to the invention. For this purpose an electrostatic latent image is developed e.g. with an aqueous dispersion of a hydrophobic binding agent and the moist image then transferred to a gelatin foil and dried. After wetting the elatin and rubbing with printing ink, only the areas covered with binding agents accept ink. Suitable dispersions of hydrophobic binding agents are latexes e.g. polystyrene latex, latex of co(butadiene/acrylonitrile) and Lytron S10 (trademark for a thermoplastic copolymer of styrene marketed by Monsanto Chemical Company, Springfield, Mass., U.S.A.).

FIGS. 5 and 6 represent devices for use in developing images according to the invention.

In FIG. 5 a roller 10 with capillary passages for sucking the ink 6 from a container 9 which is filled with the aqueous ink 6 rotates in said container. On this roller the charge bearing insulating foil 5 with a hydrophobic surface is tangentially moved forward while it is pressed by the pressure roller 11. The roller 10 and 11 may be of rubber, plastic material or metal. The output of the liquid can be regulated with a roller or a doctor knife 17.

FIG. 6 represents a developing method according to which the surface to be developed faces upwards and the ink roller 13 with capillary apertures is fed via a feed roller 15 which rotates in a container 16 filled with an aqueous liquid medium. A device 17 doses the amount of liquid taken along.

The rate of development can vary between wide limits. Good results are obtained with paper speeds between 1 cm./sec. and 1 m./sec.

When applying the development which is schematically described in FIG. 6, the application of the ink is promoted by gravity. In this case, it is preferable to use low speeds and a charge carrier with a heavily roughened surface. In the apparatus of FIG. 5, the ink has to be brought onto the surface to be developed against gravity. in this case an average speed, a relatively smooth insulating hydrophobic surface or a slight roughening of the ink roller 10 and a light pressure of the pressure roller 11 is used.

The liquid development according to the invention can be influenced by applying during the wetting step with ink, an electric field perpendicularly on the layer bearing the electrostatic charge pattern. This can be done e.g., by applying in the apparatus according to FIG. 5, a potential difference between the ink roller 10 and the pressure roller 11. A tension of 10-100 volts, the field direction of which is opposite to that of the charged insulating layer, increases the contrast. A tension of -500 volts with a field of the same direction as that of the charged insulating layer gives a counter image i.e. liquid is deposited onto the exposed areas.

By applying an alternating field perpendicular to the layer bearing the electrostatic charge pattern e.g. by connecting the rollers 10 and 11 in FIG. 5 to the terminals of an alternating voltage source, the image quality can be improved. Both the magnitude of the alternating current voltage and the frequency affect the results. If at a relatively low frequency e.g. of 50 cycles, a little ink is found to deposit on the unexposed areas, the soiling of the image background can be avoided or reduced by increasing the frequency.

Counter images may moreover be made according to the following method. For this purpose one proceeds as represented schematically in FIG. 7. A latent electrostatic image (FIG. 7a) is developed according to the invention with an aqueous solution, emulsion or dispersion (7b) which after drying leaves a hydrophobic residue (70) which is itself electrically conductive. For this purpose an aqueous dispersion of modified parrafin as marketed by Moibil Oil AG of Hamburg, Germany, under the trademark Mobilcar A is suitable. Afterdrying charging is repeated once again (7d). Only the uncovered areas of the charge carrier retain electrostatic charges. Development now takes place with an aqueous dyestuif solution (7e) which wets only the areas made hydrophilic by the previous charging.

Ink images obtained according to the invention may be transferred onto another support in a simple way, by bringing the still wet ink image in contact with an inkaccepting surface e.g. a porous surface such as paper or a transparent foil covered with gelatin. This process permits e.g. to reproduce documents according to the reflectographic exposure method, whereby a positive mirror image is obtained which is transferred onto another support so as to form a positive legible image.

Finally it has to be mentioned that the still wet ink image may be transferred onto the back-side of the support by rolling up the developed image still wet around a cylindrical rod of small diameter. In this way an ink print is transferred onto the hack-side of the wetted material. This method is especially interesting to obtain a mirror image of a text. In this method good results have been obtained, especially when using those electrophotographic materials according to the present invention which comprise a thin paper support with a certain surface roughness.

The transferof a colour image to a transparent sup port in the presence of a mordanting agent for the dyestuff is specially suitable for producing images in more than one colour. Suitable dyestuffs and mordanting agents for this process are described e.g. in US. patent specification 1,121,187. The procedure is the following: three colour separation records are formed by exposing three sheets of electrophotographic material to a coloured original through a cyan, one through a magenta and the other through a yellow filter.

Photoconductive substances the spectral sensitivity of which is suited for the manufacture of electrophotographic materials used for the reproduction of colour images are e.g. seleniumto which arsenic or tellurium are added or zinc oxide to which sensitizing agents are added as described in United Kingdom patent specifications 885.718, 895,723, 954,017, 954,018, 885,715 885,716 and 885,717. The three selection images are developed according to the invention with an ink which has the same absorption spectrum as the used filter. These ink images are then successively transferred in exact register to the same foil, so as to obtain a true colour reproduction of the original.

The invention may further be applied successfully in different printing techniques e.g. in hectography, oifset printing and for obtaining etch reliefs. In a hectographic process, a latent electrophotographic image is developed with a concentrated dispersion of a dyestuff e.g. an aqueous dispersion of crystal violet or malachite green. After drying, the developed image is employed in an alcohol duplicating machine to make the desired number of copies.

For manufacturing offset printing plates, a latent electrophotographic image is developed with a dispersion which after drying results in a hydrophobic residue. Then the material is braced on an offset machine and the image back-ground is made hydrophilic e.g. with a ferricyanide and organic acids.

For manufacturing etch reliefs, an electrophotographic material is used which in its photoconductive layer contains a polymer with free hydroxyl groups as a binding agent. The electrostatic latent image is developed with an aqueous dispersion of a substance (hardening agent) which is able to react with these hydroxyl groups. The

hardening agent either can be incorporated into the ink or dispersed in the binding agent and a catalyst for promoting the hardening reaction may be dissolved in the ink. Suitable hardening agents for these polymers are e.g. urea derivatives such as dimethylol urea described in the British patent specification 580,275, or monocyclic ureas, described in United Kingdom patent specification 575,260. After drying, the electrophotographic material is heated for a short While at 100-150 C. Hereby a reaction takes place on the image areas between the binding agent and the hardener, whereby on these areas the solubility of the photoconductive layer is diminished markedly. Now the non-hardened image parts can be removed with a suitable solvent whereupon the metal support can be etched through the etch resist in a known way.

The following examples illustrate the present invention.

EXAMPLE 1 To 500 cc. of a 4% solution of copoly(vinyl acetate/ vinyl stearate) (/15) in. ethanol, 225 g. of photoconductive zinc oxide (Neige extra pure, type A, marketed by Vieille Montagne S.A., Liege, Belgium) are added. The mixture is ground in a ball-mill for 48 hrs, whereupon the following composition is added whilst thoroughly stirring:

This photoconductive dispersion is knife-coated onto a paper support at a rate of 1 liter per 10 sq. m. and dried.

The obtained photoconductive material is charged in the known Way by a negative corona-discharge and then exposed to an image. The obtained electrostatic pattern is developed by guiding it over a developing apparatus comprising two small glass plates held at a spacing 0.4 mm. so as to form a capillary passageway. This space between the small plates is filled with an aqueous blue ink up to a height of 5 mm. from the upper edges.

On the charged areas ink is drawn out of the capillary; on the exposed and discharged areas the background remains clean.

EXAMPLE 2 On a chromium plate, having cylindrical passages of a diameter of 0.05 mm., Which passages contain ink of the following composition:

Water cc Naphtolschwarz B (Schulz Farbstofftabelle Nr.

A support with an electrostatic pattern obtained as in Example 1 is pressed. On the charged areas the ink is drawn out of the passages and is transferred onto the support.

EXAMPLE 3 A developing roller of sintered glass forming a reservolr containing capillary passages filled with ink of the following composition is used:

Water cc 100 Methylene blue (free of zinc chloride) g 2 The developing roller is then rolled over the surface of a photoconductive layer of zinc oxide dispersed in a silicone resin, which has been charged by a corona-discharge and has been exposed to an image. On the unexposed areas, ink is drawn out of the capillary passages and on the photoconductive layer a sharp positive image is obtained. The density of the image is determined by the concentration of the ink.

15 EXAMPLE 4 A support bearing a photoconductive layer is prepared as follows:

To 500 cc. of a 4% solution in ethanol of copoly (vinyl acetate/vinyl stearate) (85/15) are added 150 g. of zinc oxide having photoconductive properties. Whilst thoroughly stirring 10 cc. are added of a 10% solution in ethanol of copoly(vinyl acetate/vinyl stearate) (85/15).

Before coating, 10 cc. of a 1% solution of Rhodamine B (C.I. 45,170) in ethanol are added whilst thoroughly stirring.

The obtained photoconductive composition is applied onto a baryta-coated paper support of 90 g./sq. In. by a knife-coating system. The dried layer contains 16 g. of photoconductive zinc oxide per sq. m. The photoconductive layer is charged by corona-discharge. The original to be copied is brought into contact with the charged zinc oxide layer. The whole is reflectographically exposed through the support of the zinc oxide layer. A latent electrostatic image is obtained which is developed accord ing to the process described in Example 1 with a liquid of the following composition:

Water cc 100 Methylene blue g 2 Glycerol g 2 A strong image is obtained which remains wet for a sufiicient period as to press it against a sheet of typewriting paper. In this way a legible positive image is obtained.

EXAMPLE 5 A latent electrostatic image is reflectographically obtained as described in Example 4.

This image is developed according to the process described in Example 3 with a liquid of the following composition:

Water cc 100 Methylene blue g 2 Carboxymethylcellulose g 2 The further manipulations can be carried out as set forth in Example 4.

EXAMPLE 6 Onto a support provided with a photoconductive layer as in Example 4, an electrostatic image is applied through a metal pattern by a corona-discharge of 6000 volts in the following way: the support of the photoconductive layer is laid upon a conductive base with the photoconductive layer turned upwards. The metal pattern is laid onto this photoconductive layer. The corona-electrode is placed above the metal pattern. The tension is now applied between this electrode and the conductive base. Such a process is described e.g. by Selenyi, Z. Techn. Physik, 1935, No. 12, pp. 607-614. The charge image is then made visible using the developing method described in Example 3.

EXAMPLE 7 The photoconductive material described in Example 6, is used for recording information in the following way.

A tension of 1000 volts is applied between a conductive plate over which the recording material is moved, and a recording pin which is moved over the photoconductive layer of this material. A charge image is thus formed which is then developed to a sharp image as described in Example 3.

EXAMPLE 8 For preparing a photoconductive dispersion the following products are thoroughly mixed in a ball-mill:

Plexigum 25 (10% solution in acetone of a polyacrylate resin marketed by Rohm & Haas,

G.m.b.H., Darmstadt, Germany) cc 50 15 4% solution in methylene chloride of Vinnapas B 500/40 VL (trademark for a polyvinyl acetate marketed by Wacker-Chemie G.m.b.H., Munich,

Germany) cc 25 Acetone cc 25 Zinc oxide of Example 1 g 22.5 10% solution of monoand di-isopropyl orthophosphate (1:1) in ethanol cc 1.5

The resulting dispersion is applied to an aluminum foil by dip-coating and in such a way that the quantity of zinc oxide amounts to 15 g./sq. m.

After drying, the photoconductive layer is charged up to 400 volts by means of a corona-discharge of 6000 volts and then exposed for 1.5 sec. through a diapositive with an incandenscent lamp of watts placed at a distance of 10 cm. Development is carried out in an apparatus as schematically represented by FIG. 5.

This apparatus comprises two rollers and a container for an aqueous liquid developing medium. Roller 11 is an aluminum cylinder with a diameter of 15 mm. and a length of 25 cm., which accomplishes only the function of guiding roller for the material to be developed. Roller 10 is a chromium-nickel steel 18/8 cylinder with a diameter which is twice as large as that of roller 11. The surface of roller 10 is helically grooved in a Way that neighbouring groove convolutions are immediately adjacent. The groove has a V-shaped section the width and depth of the groove each being 0.5 mm. This roller 10 rotates freely in the developing liquid so that the groove is filled with such liquid. The liquid is removed from the crest portions of the roller, between the groove convolutions, by means of a scraper 117, or a sponge, whereas the remainder of the liquid is held in the groove by capillary forces.

The photoconductive material is conducted between the two rollers at a speed of 3 m./min., the latent image facing the grooved roller. The developing liquid consists of:

Cc. Aquablack 15 (trademark) 30 Water 100 For enhancing the contrast between the exposed and the unexposed areas the roller 11 is energized by a -10 v. tension in respect of roller 10.

A positive image is obtained.

EXAMPLE 9 The following two compositions A and B are separately ground for 24 hrs. in a ball-mill.

Composition A Zinc oxide of Example 1 g 150 4% solution of Flexbond D13 (trademark for co- (vinyl acetate/vinyl stearate) (/15) marketed by Colton Chemical Cie, Cleveland, Ohio) in ethanol cc 300 10% solution of monobutyl phosphate in ethanol cc 1O Composition B Ferric stearate g 1 4% solution of Flexbond D13 (trademark) in ethanol cc A coating dispersion is prepared: composition A is diluted with 600 cc. of a 4% solution of Flexbond D13 (trademark). Whilst thoroughly stirring, 10 cc. of a 10% solution of malonic acid in dimethyl formamide are added to said composition, whereupon the whole composition B is added with thorough mixing.

This photoconductive dispersion is applied onto an ordinary photographic paper support by dip-coating in such a way that after drying, the content of zinc oxide in the layer amounts to about 17 g./ sq. m.

Water cc 100 Pyrogallol -g 2 Sodium sulfite g 5 After drying, the image is almost invisible. By heating it at about 100 C. for a short while, a deep-blue positive image is obtained.

EXAMPLE A dispersion of the following composition is prepared and ground for 48 hr. in a ball-mill:

Siliconharzlosung RE (trademark for a solution of a phenylmethyl silicon resin in a mixture of cyclohexanone and toluene, marketed by Wacker- Chemie G.m.b.H., Munich, Germany) cc- 250 Toluene cc.. 750 Zinc oxide as in Example 1 g 300 Before applying this dispersion by dip-coating onto a baryta-coated photographic paper support, 2 cc. of a 1% solution of Rhodamine B (C.I. 45,170) are added.

After drying, the obtained photoconductive layer is charged to a tension of --300 v. by means of a coronadischarge of -6000 v. on the wires.

Next the photoconductive layer is exposed for l minute through a micro-film negative fitted in a Leitz Focomat (trademark) IIA colour enlarger by means of a 75 w. bulb so as to obtain a six times linearly enlarged image. The resulting latent electrostatic negative image is developed with the following solution:

Cc. Aquablack U (trademark) 20 Aquablack (trademark) Water 100 The development is carried out as described in Example l with the difference that a potential difference of 200 v. is applied onto the developing liquid with respect to the supportof the photoconductive layer. A positive legible image is obtained. In order to enhance the conductivity of the support, it can be moistened with water before development if desired.

EMMPLE ll Onto a baryta-coated paper support, a photoconductive layer is applied, which has been prepared as in Example 1, but by using Rose Bengale (C.I. 45,440) as a sensitizer.

In the same way as in Example 10, a latent electrostatic image is produced in said photoconductive layer. This image is developed as in Example 1, but with the following developer solution:

Cc. Mobilcer A (trademark for a dispersion of a modified paraffin, marketed by Mobil Oil A.G., Hamburg, Germany) 50 Water 50 is held in inclined position, a small amount of dyestuff solution of the following composition:

Cc. Aquablack U (trademark) 40 Water A legible positive image is obtained.

EXAMPLE 12 A photoconductive dispersion prepared as in Example 1 but without using the fiuorescein solution is applied onto a baryta-coated paper of 55 g./ sq. m. at such a rate that in the eventual coating 16 g. of photoconductive zinc oxides are present per square meter of the paper support. The zinc oxide layer is electrostatically charged and a positive latent electrostatic image is obtained. Before or during the development of this latent image the back side of the support is moistened by rubbing it with a felt cloth wetted with water to render the paper support very conductive.

Then the latent image is rendered visible by means of the developing apparatus used in Example 1 by means of the following dyestuif solution:

Water cc 100 Rayon Black G (Cl. 35,255) g 2 A positive legible image is obtained.

EXAMPLES 13-20 Images of other colours are obtained, when in the developing solution used in Example 13 Rayon Black G is replaced each time by the following dyestuffs:

C.I. Naphthalene Blue Black C Acid Black 41 Chlorantine Red 8 EN 23,050 Solar Violet 2 B 27,905 Chrome Fast Yellow GI 25,100 Naphthol Blue Black 20,470 Brilliant Carmine L 24,830 Brilliant Benzo Green Fast BL 28,455 Polar Red 3 B 24,810

Similar results are also obtained by replacing the water content up to 20% by ethanol or formamide.

EXAMPLE 21 A baryta-coated photographic paper is coated by knife-coating with a dispersion consisting of 1 part by volume of the composition A and 3 parts by volume of the composition B. 7

Composition A 6% solution in methylene chloride of co(N-vinylcarbazole/ethyl acrylate) prepared as described in United Kingdom patent specification 964,875 cc Zinc oxide (Neige extra pure, type A, marketed This dispersion is applied in such a way that each liter covers 12 sq. m. of paper. This photoconductive material is charged to a tension of 400 v. by a corona discharge of -6000 v. A positive original is episcopically projected on the charged Zinc oxide layer by means of a Fotoclark reproduction camera by an exposure of 25 sec. with bulbs to obtain a total power of 1800 w. (The Fotoclark re- Methylene Blue (C.I. 52,015) g 2 Water cc 100 with the device of Example 1.

Immediately thereafter the developed layer is brought into contact with a porous paper surface. The not yet dried ink is absorbed by the pores of the transfer paper wherein a legible image is formed.

EXAMPLE 22 The photoconductive layer described in Example 21 is applied onto a paper sheet which is provided with an aluminum foil. A latent electrostatic mirror image is formed onto this layer. Development occurs by applying the developing liquid of Example 21 using the same device as in Example 8. After drying of the ink, a master sheet for alcohol duplicating is obtained. Fifteen prints may be obtained.

EXAMPLE 23 A mixture of the following composition is prepared:

Vinnapas B 50/25 VL trakemark for a ployvinylacetate marketed by Wacker-Chemie G.m.b.H., Mu-

nich, Germany) g 3 Methylene chloride cc 33 Ethanol -cc 67 To this mixture are added 15 g. of photoconductive zinc oxide and 1 cc. of a alcoholic solution of monobutyl phosphate. The obtained mixture is ground for 16 hr. in a ball-mill. The photoconductive dispersion thus obtained is applied onto a lbaryta-coated paper of 80 g./ sq. m.

After charging by means of a corona-discharge of -6000 v., an electrostatic mirror image is formed. The charged surfaces are wetted with the following developing dispersion by means of the device of Example 8:

Ce. Aquablack (trademark) 40 Water 100 After drying the ink on the photoconductive layer, the latter is brounght into intimate contact with a sheet of common writing paper and the sheets are carried together between two smooth rollers on which a pressure of 30 kg./sq. cm. is exerted. A legible positive image is obtained on the transfer paper.

EXAMPLE 24 The photoconductive layer of Example 1 is applied onto an aluminum sheet for use as an offset plate. A latent positive electrostatic image is formed on this plate as in Example 8. The development of this image occurs also as in Example 8 by means of the following liquid:

Cc. Mobilcer A (trademark) 50 Water 50 This liquid gives a positive image, which after drying is hydrophobic and absorbs offset printing inks. The uncovered parts of the zinc oxide layer are treated with the following hydrophilic-rendering composition:

Gum arabic g 1 Trisodium phosphate g 1 Water cc 100 An offset plate ready for printing is obtained.

20 EXAMPLE 2s The following composition is ground for 24 hr. in a ball-mill:

Zinc oxide (from Example 1) kg 4.5 4% solution of copoly(vinyl acetate/vinyl stearate) (/15) in ethanol litres 9 After grinding the mixture is diluted with the following mixture:

4% solution of copoly(vinyl acetate/vinyl stearate) (85/15) in ethanol litres 21 10% solution of monobutyl phosphate in ethanol cc 300 10% solution of succinic acid in dimethyl formamide cc 300 1% solution of fiuorescein in ethanol cc 300 G. Methylene blue 2 Water By means of the apparatus described in Example 8 while an alternating tension of 100 v. and 50 cycles is applied between the rollers 10 and 11 during the development, a positive legible image is obtained At 50 cycles a vigorous image is obtained but a little ink is also deposited upon the non-exposed areas. This undesired ink-deposit can be removed by increasing the frequency of the applied tension during development, so that already at 1000 cycles a positive image with a pure background is obtained.

What is claimed is:

1. A process of developing an electrostatic charge pattern carried by an electrostatic charge carrier having a hydrophobic surface, which comprises bringing said charge carrier surface with the charge pattern thereon into contact with the ends of a closely spaced array of capillary passageways in a developer applicator, said array extending across the width of the charge carrier to be developed with the passageways being coterminous at the ends thereof in contact with said carrier surface, while maintaining the opposite ends of said passageways in communication with a supply body of a conductive polarizable aqueous developing liquid, said passageways having the entirety of their surfaces wettable by said developing liquid so as to form continuous capillary paths from said supply body to the coterminous ends whereby developing liquid is drawn from said supply body by capillary action into said passageways to form concave menisci adjacent the ends of said passageways contacting said carrier surface, the charges in said electrostatic charge pattern modifying the surface tension of said developing liquid and causing the same to flow from said passageways into wetting contact with said carrier surface according to said charge pattern, whereby developing liquid flowing out of said passageways is replaced by liquid drawn from said supply body through said passageways.

2. The process of claim 1 wherein said developing liquid is an aqueous dispersion.

3. The process of claim 1 wherein said charge carrier comprises a photoconductive layer formed essentially of finely divided photoconductive zinc oxide dispersed in a hydrophobic binder.

4. The process of claim 1 where said array of capillary passageways is formed in electrically conductive material.

5. The process of claim 1 wherein said charge carrier is normally insulating and a direct current electrical field is created across said carrier bet-ween said conductive developer liquid and an electrode on the opposite side of said carrier, said field being substantially uniform and extending in a direction normal to the carrier surface and of opposite polarity to the field resulting from said electrostatic charge pattern.

6. The process of claim 1 wherein said charge carrier is selected to have rectifying properties and an alternating current electric field is created across said carrier between said developer liquid and an electrode on the opposite side of said carrier, said field being substantially uniform and extending in a direction normal to said carrier surface.

7. The process of claim 6 wherein said alternating current field has a frequency in excess of 500 cycles.

8. The process of claim 5 wherein said field has a potential of about -500 volts.

9. The process of claim 6 wherein said field has a potential of about 10-500 volts.

10. The process of claim 1 wherein said developing liquid consists of at least 60% water and has a high conductivity of at least 10- mho/cm.

References Cited UNITED STATES PATENTS 2,556,550 6/1951 Murray 96-1 X 2,784,109 3/1957 Walkup 96-1 X 2,868,760 1/ 1959 Staicopoulos 260-41 2,891,911 6/1959 Mayer et a1 1l717.5 X 2,892,709 6/1959 Mayer 1l7-l7.5 X 2,898,279 8/1959 Metcalfe et al. 204-81 2,913,353 11/1959 Mayer et al. 117-37 2,914,403 11/1959 Sugarman 1l7-17.5 X 2,919,191 12/1959 Walkup 96-1 2,952,536 9/1960 Kurz 96-1 2,957,765 10/ 1960 'Resetich 96-1 22 6/1961 Crumley et al. 96-1 9/1961 Kurz 96-1 9/ 1961 Metcalfe et al. 117-37 10/1961 Olson 117-37 11/1961 Ricker 117-37 7/1962 Mayer 117-17.5 X 9/1962 Schaffert 118-637 9/1962 Greig 96-1 9/1962 Reithel 117-37 X 9/1962 Greig 117-37 12/ 1962 Gundlach 117-37 2/1963 Greig 117-37 3/1963 Claus 96-1 4/1963 Gundlach 117-37 7/1963 Schaffert 117-37 X 8/1963 Metcalfe et al. 117-37 10/1963 Reithel 117-37 X 12/1963 Cassiers et al. 96-1 2/1964 Middleton et al. 96-1 5/1964 Wright 117-120 X 7/1965 Blake et al. 96-1 8/1965 Herrick 117-120 X 1/ 1966 Castle 9-6-1 X FOREIGN PATENTS 10/1951 Great Britain. -6/ 1958 Great Britain.

OTHER REFERENCES Bikerman: Surface Chemistry, pp. 422-423, 1948.

Schwartz, Perry: Surface Acetone Agent, 1949, pp.

WILLIAM D. MARTIN, Primary Examiner EDWARD J. CABIC, Assistant Examiner US. Cl. X.R. 

